Anecdotal clinical experience, limited uncontrolled clinical reports, and animal studies have suggested that systemic anaphylactoid reactions to injected contrast media can be prevented by pretreatment with adrenalcorticosteroids. We have undertaken a multi-institutional study that will eventually provide us with a sufficient number of patients to test the efficacy of pretreatment with adrenalcorticosteroids in comparison with placebo. Several different regimens have been designed for the steroid or placebo pretreatment. Patients thought to be particularly at high risk because of a previous history of reactions to contrast media have been designated to receive steroid pretreatment. All other patients have been randomized to either a one- or two-dose steroid (32 mg Medro or placebo regimen). It is projected that somewhere between 30,000 and 50,000 patients will have been entered into the study at the time this application could be activated. Present projections suggest that perhaps 50,000 more patients will be necessary.